This invention relates to a simple, compact, and robust, cylindrical microlens external cavity for laser diode frequency control. The microlens system makes possible a tunable laser diode that utilizes an orthogonal pair of collimating microlenses to form a short external cavity for longitudinal mode and frequency control. The resulting system generates a circular, collimated beam of single frequency, tunable laser radiation.
Keywords: tunable laser diode, external cavity, microlens, laser diode frequency control, optical feedback.
A typical edge emitting laser diode consists of a crystal with polished end facets and electrodes to supply the bias. The reflectivity of the uncoated facets, due to the high index of refraction of the diode material, is of the order of 30%. This low reflectivity, coupled with the high gain necessary to overcome such losses and allow the production of coherent light means that any light which scatters or reflects from external optical elements (or even the laboratory walls) will find its way back into the laser and affect its operation. Depending on its magnitude and phase, light which has been fed back into a laser diode may degrade its output by causing multimode operation or a broadening of its linewidth, or it may enhance single mode operation and narrow the linewidth. (See G. P. Agrawal and N. K. Dutta, Long-Wavelength Semiconductor Lasers(Van Nostrand Reinhold, New York, 1986))
Most laser diodes can be tuned over a substantial range by changing either the temperature or the drive current. However, the tuning is typically not smooth and not continuous. An example of the dependence of a diode's output wavelength as a function of the current or the temperature is shown in FIG. 1a and FIG. 1b. (See J. C. Camparo, "The laser diode in atomic physics," Rev. Sci. Instrum. 62, 1-20(1991)). FIG. 1a shows a typical temperature tuning curve for a laser diode. A single mode of the laser can tune as much as 0.4 nm by changing the temperature before "hopping" to another longitudinal mode. Similarly, as shown in FIG. 1b, the wavelength of the laser can be tuned by varying the current. In that case the wavelength can change by about 0.1 nm before mode hopping. While there are regions in which the wavelength changes smoothly, in between these regions are abrupt changes which correspond to longitudinal mode "hops" described above. For the case of temperature tuning these hops are caused by the faster change of the laser's gain curve with temperature than its change in mode frequency (due to the change in optical length). Unfortunately for users of laser diodes, the tuning range without external feedback will have substantial gaps; wavelength bands the diode cannot access. Although there are other, more complex laser diodes, such as distributed feedback (DFB) lasers, which are intrinsically single mode and can be tuned Continuously over wider spectral regions, they are more expensive and not readily available at most laser diode wavelengths.
Without optical feedback, the laser can be current-tuned as much as 50 GHz (0.1 nm) as long as its frequency is not near a mode hop and the accompanying variation in optical power is acceptable. The operating wavelength can be coarsely tuned by as much as 10 or 20 nm by changing the temperature. Thus with a combination of current and temperature tuning, one might expect that a laser diode can be tuned to any wavelength that falls within its gain bandwidth.
In practice, because of the mode hops, not every laser will operate at a particular desired wavelength. It has been experimentally shown, however, that optical feedback can suppress mode hops in a laser diode's tuning curve. (See A. Hemmerich, et al "Optically stabilized narrow linewidth semiconductor laser for high resolution spectroscopy," Opt. Comm. 75, 118-122(1990)) It does so by enhancing the gain conditions of one of the longitudinal modes over the others. It is therefore generally accepted that some form of optical feedback mode control is necessary to obtain an all-purpose tunable laser diode. (See CI.E. Wieman and L. Hollberg, "Using laser diodes for atomic physics," Rev. Sci.lnstrum. 62, 1-20(1991)).
A common way of ensuring single mode operation in a laser diode is to create a short external cavity by placing an optical surface 100 microns or less from the laser output facet. This has been done both with a flat piece of glass and with a GRIN lens. (See G. P. Barwood, et al, "Longitudinal mode control in laser diodes," Meas. Sci. Technol. 3, 406-410(1991)) Short external cavities based on experimental lasers in which both outputs of the laser are accessible have also been investigated. (See D. T. Casidy et al, "Short-external-cavity module for enhanced single mode tuning of InGaASP and AlGaAs semiconductor laser diodes," Rev. Sci.lnstrum. 62, 2385-2388(1991)) In this case a flat or spherical mirror has been employed to reflect the light exiting one end back into the laser while leaving the other output unobstructed. However, such lasers are not always easily obtainable. Also, many commercially available laser diodes are provided in a package in which only one output is accessible.
Prior published work on laser diodes with short external cavities based on graded index (GRIN) lenses has reported a frequency pulling range of 2 GHz. The GRIN lens external cavity provided sufficient control to allow continuous temperature tuning of the laser diode over a wavelength range of 2 nm with the external cavity length fixed.
The value of the short external cavity concept is that it can be used in conjunction with broad-band tuning techniques either for simple longitudinal mode selection, or alternatively as a fine control for sweeping or dithering the frequency of the laser. However, short external cavities based on thin glass plates or on GRIN lenses have low efficiency because the light reflected back to the laser is strongly divergent. To compensate for the geometric losses of the cavity due to divergence, the reflectivity of the surface must be high, so the useable optical power is low.
What is needed is a system which provides sufficient feedback but transmits the major fraction of the output, so that the laser can be tuned over a substantial frequency range while avoiding mode hopping.